[SEL] Foundry Question

[peter ogborne]

This is a difficult question ,at least I mean getting the correct answer .. I have friend who has had quite good success with his home foundry . To date he has cast alluminium ,brass and bronze and a small number of cast iron jobs .He cast two CI bearing caps for me and they machined well and the castings are first class. His home made furnace is fueled by LPG with an air blower . The furace itself is a crucible with flame entering at a tangent . There has been no great problem with noise apart from the normal ''Roar'' of the blower and flame whilst melting Alluminium ,Brass or Bronze . With higher temperature required for the Iron melt comes a high frequency scream  It alternates in its volume ..Unfortunately this noise now presents a neighbor problem.It only occurs with the higher temp required for the iron.
I know this is not an explicit explantion of the symptons but has any one experienced anything like this ?